1. Field of the Invention
The present invention relates to a multilayer wiring base board, and, in more detail, relates to a base board constitution having a coaxial structure with plural cores, a method of producing thereof and an electronic device.
2. Description of the Related Art
For a wiring base board which transmits a high-speed signal, differential transmission by means of paired wiring is used as transmission system which is strong in an external noise. In case in which a coaxial via intended for conventional single transmission is used for such a differential transmission system, a differential structure is not obtained at this via part.
For example, in connection with this coaxial via structure, an explanation is given by referring to FIG. 1 through FIG. 3. FIG. 1 is a drawing showing a part of an upper face of a base board which has the coaxial via structure, FIG. 2 is a longitudinal sectional view of a part taken along line II-II of FIG. 1, and FIG. 3 is a transverse sectional view of a part taken along line III-III of FIG. 2. In this coaxial via structure, a via hole part 4 is formed in a wiring base board 2 as a single signal hole. Inside of this via hole part 4, a GND (grounded conductor) wall 6 is provided, and a via conductor part 10 which constitutes a via core forming a coaxial structure with the GND wall 6 through an insulator 8 is also provided. The via conductor part 10 is connected to a wiring pattern 12 of an upper face side of the wiring base board 2, and is also connected to a wiring pattern 14 of a lower face side of the wiring base board 2.
In cases where a guard pattern is provided in such a coaxial via structure, an explanation is given by referring to FIG. 4 through FIG. 6. FIG. 4 is a drawing showing a part of an upper face of a base board which has a coaxial via structure, FIG. 5 is a longitudinal sectional view of apart taken along line V-V of FIG. 4, and FIG. 6 is a transverse sectional view of a part taken along line VI-VI of FIG. 5. In this coaxial via structure, a GND guard pattern part 16 which surrounds the wiring pattern 12 is formed in an upper face part of the wiring base board 2. This GND guard pattern 16 and the GND wall 6 are electrically connected by forming a plurality of non-through holes 18 in a lower face side of the GND guard pattern part 16 and providing a conductor part 20 inside each non-through hole 18.
In connection with a wiring base board which uses such a coaxial via structure for differential wiring, an explanation is given by referring to FIG. 7 and FIG. 8. FIG. 7 is a differential wiring structure which does not use a guard pattern, and FIG. 8 is differential wiring which uses a single-core coaxial via part. Differential wiring 26 is constituted by a pair of via hole parts 22 and 24. Wiring patterns 23, 25, 27 and 29 which constitute the differential wiring are connected to each of the via hole parts 22 and 24. In the differential wiring 26, one signal is inverted into a positive phase (+) side and an opposite phase (−) side and is transmitted as two signals, and, by both of them having an influence on each other through electromagnetic coupling, the improvement of a transmission characteristic is given. However, as shown in FIG. 8, each of the via hole parts 22 and 24 is surrounded by the GND wall 6, and thereby, each of coaxial via hole parts 31 and 33 is constituted. In the coaxial via hole parts 31 and 33 of a constitution like this, even if the two via hole parts 22 and 24 are parallel conductors, the differential wiring is not obtained because the electromagnetic coupling is interrupted by the GND wall 6. That is, the signal transmission of the coaxial via hole parts 31 and 33 is nothing but single-wire transmission.
As to patent documents regarding the signal transmission of a via part like this, there are the Japanese Patent Laid Open Publications No. 2002-353588, No. 2003-243831 and so on.
The publication No. 2002-353588 discloses a wiring base board which uses differential wiring. According to this wiring base board, in relation to a through hole (a via hole part) which transmits a differential signal on a multilayer wiring base board, a first through hole is formed. Further, an insulator part is formed by filling the first through hole with an insulating resin, and a pair of second through holes which transmits the differential signal is formed in the insulator part.
Further, the publication No. 2003-243831 relates to a wiring base board and its producing method, and discloses the following wiring base board. This wiring base board comprises a metal core substrate having a surface and a back face, an elliptic (non-circular) through hole formed in the metal core substrate, and a pair of through hole conductors arranged in the through hole though an insulating material. The through hole is an elliptic shape which surrounds the pair of through hole conductors, and a distance between an inner wall of the through hole and an outer circumferential part of an opposing through hole conductor out of the through hole conductors is made non-uniform.
By the way, the wiring base board disclosed in the publication No. 2002-353588 has the following constitution. As shown in FIG. 9, a large-diameter conductor part 32 is formed in a via hole part 30, and conductor parts (plating parts) 36 and 38 are provided inside the conductor parts 32. Further, by providing an insulator 34 between these conductor parts, differential wiring is given. In the publication No. 2002-353588, it is mentioned to be a coaxial structure. However, a center Oa of the conductor part 32, a center Ob of the conductor part 36 and a center Oc of the conductor part 38 are set at different positions. Assuming that a distance (radius) from center Ob or Oc to outer face part of the conductor parts 36 or 38 is r, a distance R (=R1, R2, R3, . . . ) from each of the centers Ob and Oc to inner face part of the conductor part 32 differs by means of an angle θ (R1‡R2‡R3‡ . . . ). That is, a distance between an outer circumferential face part of the conductor parts 36 or 38 and an inner circumferential face part of the conductor part 32 differs by an angle. It is not said that the conductor part 32 and the conductor parts 36 and 38 differing in a center position as described above are the coaxial structure.
Furthermore, in the wiring base board disclosed in the publication No. 2003-243831, the metal core substrate provides the elliptic through hole and nothing more. Since the core substrate is a metal plate, a connecting structure between a surface and a back face of the base board is restricted. Further, if the metal plate is made thick, it becomes heavy, and a restriction is placed on multilayer wiring within a width of a thickness of the metal plate.